1. Field of the Invention
This invention relates to display devices, and particularly to a clamping circuit for securing a minimum reference voltage of a boost regulator with a variable reference input in a display device.
2. Description of the Related Technology
Recently, there has been a great deal of development in the area of small flat-panel displays which require low power and are generally used for PDAs (Personal Digital Assistants), cellular telephones and automobile instrumentation, for example.
An OLED (Organic Light Emitting Diode) display or a PLED (Polymer Light Emitting Diode) is a well-known example of such small flat-panel displays. The OLED display is becoming widely used because it has many advantages such as low power consumption, full-color and wide viewing angle. Unlike a Liquid Crystal Display (LCD), the OLED is a current driven device. However, it is similarly arranged in a 2 dimensional array (matrix) of pixels to form a video display.
FIGS. 1A and 1B show typical physical structures of a PLED or OLED display device (Hereinafter PLED and OLED will be referred to as PLED for convenience). A representative series of row top electrodes 110, which include parallel conductors 111–118, are disposed on one side of a sheet of light emitting polymer 120. A representative series of column electrodes 138 that include parallel transparent conductors 131–138 are disposed on the other side of a light emitting polymer sheet 120, adjacent to a glass plate 140. Referring to FIG. 1B, a display cross-section 100 shows a drive voltage V 160 applied between a row 134 and a column 111. The potential developed between the row 111 and the column 134 across the thickness of the sheet 120 causes current flow through the sheet 120 and causes the light emitting polymer 120 to emit light. The emitted light 170 passes through the column conductor 134 which is transparent.
This structure results in a matrix of PLEDs, one PLED formed at each point where a row overlies a column. There will generally be M×N PLEDs in a matrix having M rows and N columns. Typical PLEDs function like light emitting diodes (LEDs), which conduct current and emit light when a voltage of one polarity is applied across them, and block current and stop emitting light when a voltage of the opposite polarity is applied. Exactly one PLED is common to both a particular row and a particular column, so as to control these individual PLEDs located at the matrix junctions. The PLED display device generally has two distinct driver circuits, one to drive the columns and the other to drive the rows. It is conventional to sequentially scan the rows (typically connected to the PLED cathodes) with a driver switch to a known voltage such as ground, and to provide another driver, which may be a current source, to drive the columns (which are typically connected to the PLED anodes).
A boost regulator is a circuit that automatically adjusts the amount of current flowing through a load in order to maintain a constant output voltage. The boost regulator performs such a function by comparing a reference voltage and an output sample voltage and generating a difference voltage between the two. A feedback control loop adjusts the regulator current output to minimize this difference, thereby achieving a constant output voltage. The boost regulator is used in many electronic devices.
The boost regulator is also used in the PLED display device and generates a drive voltage for the current source of the PLED display based on an input reference voltage. In some situations, it happens that the input reference voltage of the boost regulator is unstable or is too low so that the boost regulator can not provide a proper drive voltage for the current source.
Thus, what is needed in the art is an apparatus for providing a minimum stable reference voltage to a boost regulator.